1. Field of the Invention
The present invention generally relates to power converters and more particularly a cascaded converter capable of storing energy in a reservoir capacitor and insulating pulsating load currents from a primary energy source, such as a battery.
2. Description of the Related Art
Many of today""s wireless phones or cellular phones transmit signals in a time-division multiple access (TDMA) scheme such as GSM (Global System for Mobile Communications) or GPRS (General Packet Radio Signal). Up to eight phones share a frequency band and each phone transmits signals in a burst.
However, the operation of a TDMA phone causes a pulsating load current to be drawn from a battery source. Due to high source resistance in a battery, such as a lithium-ion battery, a pulsating load current can cause a severe voltage droop problem. In addition, pulsating load current aggravates the power loss caused by the source resistance.
FIG. 1 shows a power system for a typical cellular phone handset. A lithium-ion battery 11 supplies an output voltage VCC 13 directly to an RF Power Amplifier (PA) 14. The battery output voltage VCC 13 also supports several linear regulators such as regulators 15, 16, for a digital signal processor and a flash memory, respectively. A typical lithium-ion battery for cellular phone has a substantial source resistance. A resistance of about 0.2xcexa9, modeled by resistor Rs 12, includes the source resistance and other resistances such as fuse and battery contacts.
FIG. 2A shows the waveform of a pulsating load current drawn by a typical power-amplifier 14 for the above mentioned applications. The power-amp 14 draws about 2 Amperes (A) for about 580 microseconds (xcexcS) in burst transmission period (with a transition time of about 10 xcexcS), and rests, in an idle period, for about 4060 xcexcS. Thus, there are eight such transmission periods within the total cycle time of 4640 xcexcS, and the duty cycle of each burst is one eighth. For proper operation of the power amplifier PA 14, it is important for the voltage on node 13 to have small ripple (typically less than 0.3 V). In addition, the RF PA 14 requires a minimum operating voltage of 3.3 V to assure sufficient transmission power and communication quality.
FIG. 2B shows the waveform of the VCC node 13, whose voltage drops from about 3.5 V to 3.1 V because of the 2 A peak current that is required during a transmission interval. The 0.4V ripple exceeds the ripple requirement of 0.3 V for the RF PA 14. Furthermore, whenever the battery voltage falls below 3.7 V, VCC drops below 3.3 V during a 2 A load pulse. In other words, battery 11 cannot be used to support either the RF PA 14 or the LDO 16 if its voltage drops below 3.7 V, leaving as much as 40% of the battery""s total energy unusable.
The power loss for a pulsating current, such as is shown in FIG. 2A, is the product of the source resistance Rs and the square of the Root-Mean Square (RMS) value of the current. In this case, the power dissipated by Rs is approximately ({square root over (22/8)})2xc3x970.2xcexa9=0.1 W. Since the power consumed by the load is (2/8)Axc3x973.3 V=0.825 W, the power loss from Rs 12 amounts to about 12% of the load power.
Another drawback for a power amplifier operating on an unregulated voltage is wasted power. When a lithium-ion battery is fully recharged, it has a nominal 4.2V output voltage. Driving the PA with 4.2 V consumes,                     (                                            4.2              2                        /            8                          )            2              1.65      ⁢              xe2x80x83            ⁢      Ω        ,
or about 1.34 W over an entire transmission cycle, i.e., from the beginning of T0 to the beginning of T2 in FIG. 2A. Compared with the 0.825 W that is actually required during the transmission, about 63% of battery power is wasted in over-driving the power amplifier.
FIG. 3 shows a prior-art circuit that employs a large storage capacitor to reduce the effects of pulsating load current on a battery source with a substantial source resistance. A 4700 xcexcF capacitor 35, having an equivalent series resistance (ESR) 36 of about 50 mxcexa9, is connected in parallel with the lithium-ion battery 31, which has a 0.2xcexa9 internal resistance 32. The energy stored in capacitor 35 provides a low impedance energy source for the pulsating current of the RF power amplifier 34 and helps to reduce the voltage droop of the battery output voltage 33.
FIG. 4A shows the waveforms of the current drawn by the PA 34 in FIG. 3 and the current supplied by the battery 31. In particular, waveform 41 shows the PA 34 drawing 2 A during the 580 xcexcS transmission interval and no current outside of the interval. Waveform 42 shows the current supplied by the battery 31. The difference between the two waveforms is the current supplied by the capacitor 35. As is clear from the figure, the addition of capacitor 35 reduces the ripple and RMS value of the battery output current.
FIG. 4B shows the waveform 43 of the battery output voltage 33 with the additional capacitor 35. The battery output voltage ripple is reduced to about 0.2 V. Starting at time T0, a 2 A current flows causing about 0.08 V to be dropped across the 50 mxcexa9 ESR 36 of capacitor 35. Between T0 and T1, capacitor 35 provides the most of the load current to PA 34. Battery current increases gradually from about 0.4 A at T0 to about 1.05 A at T1, and VCC 33 drops further from 3.42 V to about 3.3 V at T1.
At T1, the load current of PA 34 drops to zero. Voltage VCC 33 jumps back to 3.38 V (due to the ESR effect). Battery output current now gradually recharges capacitor 35 back to 3.5 V at T2 to prepare for another current pulse at T2.
It is clear that adding a large capacitor in parallel to a battery reduces the ripple voltage to less than 0.2 V, and extends the usable battery voltage range to about 3.5 V from the previous 3.7V. However, a 4700 xcexcF capacitor adds significantly to the cost of the system. Such a capacitor is bulky and requires a large amount of PC board space. Furthermore, adding a large capacitor will not reduce wasted power when the battery has a high voltage (greater than 3.8V).
Thus, there is a need for a method and apparatus that uses a much smaller capacitor to reduce the ripple voltage of a RF power amplifier in a cellular phone handset, that regulates the supply voltage to the PA at 3.3V, and that avoids over-driving the PA when the battery voltage is substantially higher than 3.3V.
The present invention is directed to these needs. A method in accordance with the present invention includes providing a voltage on a reservoir capacitor by converting a voltage and current provided by a primary power source, and maintaining an average constant current supplied to the reservoir capacitor, where the capacitor voltage is greater than the power source voltage. The method further includes, while maintaining average constant current to the reservoir capacitor, converting the capacitor voltage to a predetermined output voltage for the load device, and maintaining the predetermined output voltage substantially constant, while providing a current pulse to the load device.
An apparatus in accordance with the present invention includes a reservoir capacitor, a first converter stage and a second converter stage. The reservoir capacitor is used to store energy obtained from a primary power source. The first converter stage is configured to convert the primary power source voltage to a voltage on the reservoir capacitor while maintaining a substantially constant average current to the reservoir capacitor. The second converter stage is configured to convert the capacitor voltage to an output voltage for a load device, while maintaining the output voltage substantially constant and providing a current pulse to the load device. According to a version of the invention, the first converter stage is a boost converter and the second converter stage is a buck converter. Between the two converters is the reservoir capacitor. The present invention uses relatively small reservoir capacitor to store energy at an elevated voltage. The boost converter stage essentially stretches the pulsating load current to almost a constant current (having a very small RMS to DC ratio), thus, greatly reducing the voltage droop and power loss across the source resistance of the battery.
The buck converter stage is capable of regulating the VCC voltage of the PA, by drawing power from the reservoir capacitor. Since the reservoir capacitor can store a relatively large amount of energy in the form of elevated voltage, the pulsating load current will only produce a large voltage droop on this reservoir capacitor, not on the battery.
One advantage of the present invention is that a much smaller energy-storage capacitor can be used.
Another advantage is that the load current waveform is changed from a high peak, low duty cycle waveform to a nearly constant waveform.
Yet other advantages are that (i) the power loss from the battery source resistance is reduced, thereby improving efficiency, (ii) the usable battery voltage is extended from 3.5 V to 2.8 V, which increases usable battery life, (iii) talk time is extended without over-driving the PA when the battery voltage is substantially higher than the 3.3V that is required by the PA, and (iv) increased power and thus communication range are available in an emergency situation, by setting PA""s VCC to 4.0 Volts.